Weather affects herbicide action

“If the plant systems targeted by the herbicide are not functioning properly due to stress – temperatures that are too hot or too cold, moisture levels that are too high or too low, or extreme events such as frost, hail or severe winds – the herbicide effectiveness may not be up to expectations,” says Clark Brenzil, weed control specialist for Saskatchewan Agriculture.

It is warm enough to spray when daytime high temperatures are near 20 C. For best results, night temperatures should be at least 10 C. It shouldn’t be too hot, either – daytime high temperatures should be less than 30 C.

“For good weed control and crop tolerance, herbicides rely on plants functioning at their optimum,” he said.

“Outside the temperature range of 20 to 30 degrees C, the metabolism of plants is inhibited. On the other hand, the growth of weeds slows or stops when stressed, so the time window for spraying is extended.”

Frost immediately before or after herbicide application can cause the herbicide to become trapped in the leaf, and prevent it from moving to the point of action.

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After waiting for 24 to 48 hours after the frost, producers should check the crop and weeds for symptoms of frost damage, which is a watery, blackened or brown appearance.

If the crop or weeds are severely damaged, wait to apply herbicide after new tissue begins to grow.

“If the plant survives a frost with little damage, applications should still be delayed until two days after the weather returns to spray conditions. This will allow the plant functions to return to normal.”

Weather extremes also affect the soil and, in turn, the plant and its uptake of herbicides.

Unless adapted to survive in water, for example, plant roots in saturated soil are starved for oxygen. Without oxygen, the root will suffer and the plant stops growing.

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On the other hand, during dry periods, plants create barriers so as to conserve moisture. This inhibits herbicide uptake into the weed. Dry or hot conditions also limit the plants’ metabolic activity, allowing time for some of the herbicide to deactivate.

When growing conditions are good, the crop breaks down the selective herbicide before it can hurt the crop. Under adverse conditions, the plant’s ability to metabolize the herbicide can be inhibited, causing the herbicide to have a toxic effect on the crop.

Other factors

Other inhibitors of herbicide effectiveness are late staging of the crop plant and reduced rates of herbicide. With the exception of glyphosate products, reducing water volume can lower the herbicide’s effectiveness.

Brenzil said producers should remember that using integrated weed management strategies, which are designed to improve the competitiveness of their crops, can also improve the chances of successful herbicide application.

If herbicide failures occur under good conditions with early staging, full rates and newly purchased product, producers may need to look at other reasons for a herbicide failure, such as herbicide resistance or, in rare cases, legitimate product problems.